This invention relates to magnetic inks useful for the production of magnetic recording products, particularly in the form of magnetic stripes greater than 600 oe coercivity coated on plastic or paper stock, the surface coating being comprised of ferromagnetic barium ferrite particles in a flexible bending medium comprised of a blend of high molecular weight polyurethane thermo plastic polymer and a vinyl chloride-vinyl acetate copolymer.
Magnetic stripes have found successful applications in ID badges, credit cards, parking garage tickets, retail tags, turnpike cards, and amusement park tickets. The reasons for this wide acceptance of magnetic stripes for information storage are: cost effectiveness of magnetic storage; ease of application; ease of data introduction, data transfer and information update; high data storage capacity of magnetically encoded products, versatility of recording systems available for magnetic recording and wide range of substrate available for producing acceptable striped product.
In one popular method of producing magnetic stripes for encoding information on a paper substrate, a slurry containing finely divided gamma iron oxide, vinyl resin, and a volatile solvent is applied directly to a paper substrate by a gravure coating process. The stripe is produced as a continuous coating on a roll of paper about 10,000 feet in length. Tickets are cut from the paper roll to appropriate dimensions.
To record information on the striped ticket, the ticket is placed directly against a magnetic recording head that travels across the length of the stripe at a constant speed. The magnetic stripe is subjected to a fluctuating magnetic field, the fluctuation being set up by an incoming signal. A magnetic record of the signal is thus impressed on the stripe. When the stripe is subsequently put in contact with a similar magnetic recording head connected to a reproduction circuit, the original signal is regenerated in that circuit.
The maximum number of signals impressed on the stripe by the magnetic recording head is limited by the size of the magnetic particle in the ink slurry, the coercivity of the magnetic particle, and the surface smoothness of the stripe. Reducing the size of the magnetic particle, increasing its coercivity and preparing a smoother surface results in an increase in the number of signals per unit area impressed on the stripe. The number of signals on the stripe per unit area is referred to as the recording density.
It is desirable to produce magnetic stripes with a high recording density to maximize the amount of information carried on a single stripe.
Still further requirements of magnetic recording striped products, particularly in the fields of credit cards, turnpike tickets, transit and airline passes and retail tags is to produce a stripe that cannot be intentionally or accidently altered easily. The problem of accidental erasure is most acute in applications , such as turnpike tickets, where the ticket after encoding will be placed near a magnetic field. An example of such a problem is a turnpike ticket placed in contact with the radio's speaker magnet on the dashboard of an automobile. Increasing the coercivity of the particle used in the ink decreases the potential for accidental or intentional unauthorized alternate of the signal.
Conventional inks for magnetic stripe applications employed prior to this invention were inherently defective in one or more particulars. Inks based on gamma iron oxide are low coercivity and, as such are capable of storing relatively small amounts of data. In addition, they are susceptible to accidental erasure from very low external magnetic fields.
Inks prepared from high coercivity particles such as cobalt ferrite, while not easily altered accidently, are made from ferrites of large particle size. This results in a rough surface that is unacceptable for magnetic recording. Small particle size cobalt ferrite are not magnetically stable and spontaneously demagnetize at temperatures above 30 deg. C. This makes them unsuitable for use in a magnetic storage medium.
Barium ferrite particles are of sufficiently high coercivity and small size to be useful. A further problem with barium ferrite, however, is the tendency for particles to stack during processing. When they are exposed to an orientation field. The orientation field is necessary to achieve high magnetic renenance. Particle orientation is a common practice in the production of magnetic recording products. The stacking of particles, however, causes the coercive force to increase so substantially that it becomes impossible to encode a signal on the stripe.